Fluid unmixing/immiscibility as an ore-forming process in the giant REE–Nb–Fe deposit, Inner Mongolian, China: Evidence from fluid inclusions

Abstract

The giant Bayan Obo REE–Nb–Fe deposit consists of replacement bodies hosted in dolomite marble made up of magnetite, REE fluorocarbonates, fluorite, aegirine, amphibole, calcite and barite. Two or three phase CO 2-rich, three phase hypersaline liquid–vapor–solid, and two phase liquid-rich inclusions have been recognized in mineralized fluorite and quartz samples. Microthermometry measurements indicate that the carbonic phase in CO 2-rich inclusions is nearly pure CO 2. Fluids involving in REE–Nb–Fe mineralization at Bayan Obo might be mainly of H 2O–CO 2–NaCl–(F–REE) system. Coexistences of brine inclusions and CO 2-rich inclusions with similar homogenization temperatures give evidence that immiscibility happened during REE mineralization. An unmixing of an original H 2O–CO 2–NaCl fluid probably derived from carbonatitic magma. The presence of REE-carbonates as an abundant solid in fluid inclusions shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo.